Prefilled syringes containing a chemical liquid that is previously filled therein are being increasingly used these days because they are very easy to use and can contribute to elimination of misuse. Since such a prefilled syringe includes rubber members that are to be in direct contact with a chemical liquid until use, butyl rubber-based gaskets and nozzle caps, which are excellent in chemical resistance, gas permeation resistance, water vapor permeation resistance and aging resistance, are used for these rubber members in many cases.
Some drugs such as biopharmaceuticals, however, may be affected by interaction with raw rubber of rubber materials, substances extracted from the compounding ingredients, coating agents fallen off the syringe, or the like. In the case of butyl-based rubber members, for example, an oil-type or curable silicone as a lubricant is applied to the inner wall of a syringe barrel and the rubber surfaces of a gasket and nozzle cap for the purpose of improving the slidability of the gasket and preventing firm fixing of the nozzle cap. Thus, such a silicone coating agent, when used with some types of pharmaceuticals, may fall off the inner wall of the barrel or the rubber members and thereby serve as foreign matter, possibly causing a severely harmful influence on the quality of the pharmaceuticals. In particular, the barrel inner wall is coated with a larger amount of silicone coating agent than the gasket. Thus, and in view of the area of contact with the liquid pharmaceutical, the silicone coating agent on the barrel inner wall has a greater influence.
Such a situation has led to development of products made of rubber laminated with, for example, a fluororesin film that has better pharmaceutical stability than butyl rubber. These products are offered for glass syringes or resin prefilled syringes. For example, fluororesin films with excellent chemical resistance are used; in particular, polytetrafluoroethylene (PTFE) with the lowest coefficient of friction is used.
However, a gasket with a fluororesin film laminated on its liquid-contact portion and sliding seal portion contacting with a barrel inner wall, has disadvantageous airtightness and sliding resistance. Especially, skived films have a rough surface and easily suffer poor airtightness between the film and the barrel inner wall. Examples of the factors for deteriorating the airtightness include the smoothness (surface roughness) of a laminated film and the variations of the barrel inner diameter which affect the compression ratio of the gasket when compressed to the barrel inner diameter.
Patent Literature 1 thus proposes a PTFE film produced by a casting method and a UHMWPE film produced by an inflation method; however, these production methods are special and thus impractical. Further, lamination of a PTFE film having a surface roughness Ra of 0.05 μm or lower may suffer a liquid leakage problem. This is because fine irregular marks on the surface of a mold are transferred to the surface of the laminated film and thereby affect the sealability even though the PTFE film is lamination-molded at a molding temperature of 170° C., which is lower than the melting temperature of PTFE (230° C.). The aforementioned variations of the barrel inner diameter are also a serious problem as they affect the compression ratio of the gasket when compressed to the barrel inner diameter. Resin syringes are accurate enough to have an inner diameter tolerance of ±0.1 mm. In contrast, the inner diameter of a tube for glass syringes greatly varies. For example, even a 5-ml or smaller tube has an inner diameter tolerance as large as ±0.15 mm.
Patent Literature 2 proposes a configuration in which the half of a front circular protrusion is laminated. This configuration is formed by a special method in which the unlaminated sliding portion is reduced in diameter to enter inside an upper edge of a punching blade. This method can be applied to small gaskets for 1-ml syringes, for example. However, the method cannot be used to produce large gaskets because the portion which is reduced in diameter to enter inside a punching blade becomes too thick, resulting in difficulty in punching. Further, the upper edge can be shortened by polishing, which causes a maintenance problem.
Meanwhile, a gasket having one or more circular cylindrical ribs is proposed. The arc-shaped circular rib is excellent in terms of sliding resistance but is poor in airtightness because of its small sliding contact area. Many front circular ribs each are designed to have the same diameter to be linear. In this case, the value of sliding resistance increases as the compression ratio, when compressed to the syringe barrel inner diameter, increases. This adversely affects plugging of the gasket. Conversely, if the compression ratio is set to be small, poor airtightness is caused. This is because the front rib portion is made of solid rubber and thus the rubber is difficult to deform.